Fail safe photoelectric sheet sensing machine control circuit



Dec. 26, 1967 T. BERNOUS 3,360,652

I FAIL SAFE PHOTOELECTRIC SHEET SENSING MACHINE CONTROL CIRCUIT Filed OCt. 14, 1964 2 Sheets-Sheet l I INVENTOR la. 3 TAYEB aamaous 26 BVM A TTORNEYS Dec. 26, 1967 T. BERNOUS FAIL SAFE PHOTOELECTRIC SHEET SENSING MACHINE CONTROL CIRCUIT Filed 00*, 14, 1964 2 Sheets-Sheet FIG. 4

v INVENTOR TAYEB BERNOUS BY :5

6AM arroers United States Patent Ofiice 3,360,652 FAIL SAFE PHOTOELECTRIC SHEET SENSING MACHINE CONTROL CIRCUIT Tayeb Ber-nous, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 14, 1964, Ser. No. 403,844 2 Claims. (Cl. 250-219) ABSTRACT OF THE DISCLOSURE A fail safe photoelectric circuit to shut down a xerographic apparatus in the event of paper adhering to the surface of the xerographic drum or failure of the circuit itself. Light from a lamp is reflected from the surface of the drum to a photocell. If paper adheres to the drum, too much light is reflected to the photocell and a diode circuit having negative resistance characteristics, is triggered to short a power supply which energizes a machine control relay. If the lamp, photocell, or other circuit element fails, there will be insuflicient current through the photocell to trigger an SCR which is connected in series with said machine control relay.

This invention relates to an electrical circuit and, particularly, to an electrical circuit to control a machine in response to three different levels of illumination on a photoconductive cell.

More particularly, this invention relates to a circuit for use with a photoconductive cell which detects the presence or absence of a sheet of paper on a rotating drum in a xerographic apparatus and further acts as a fail safe device in that it detects functional inoperativeness of the photocell itself. In a zero-graphic apparatus, a rotating drum having a photoconductive surface is used to create an electrostatic latent image on its surface. A powder image in the configuration of the electrostatic latent image is developed on the drum surface and transferred to cut sheets of paper by means of an electrostatic charge applied to the back of the paper. Since there is a latent electrostatic charge on the surface of the drum and an electrostatic charge is applied-on the paper during the transfer operation, the paper tends to adhere to the surface of the xerographic drum. The sheets of paper are stripped from the surface of the drum after transfer, usually by a putter which blows a sharp stream of air between the paper and the drum and physically forces the sheet of paper away from the drum surface. Occasionally, a sheet of paper will adhere to the drum surface past the position at which it should be removed. This occurs when, for any reason, the puffer mechanism fails to operate or the paper is not aligned in the proper position on the drum to be removed from the drum. When this situation occurs it of paper on the drum surface and immediately shut downthe machine before subsequent copies are reproduced or the paper is jammed within the machine.

The present-invention permits the utilization of alight source directed against the surface of the xerographic drum with the light reflected into a photoconductive cell to detect the presence or absence of paper on the drum surface and to shut down the machine upon the detection of a sheet of paper. It also functions to shut down the machine when the photoconductive cell itself becomes inoperative as, for example, the glass on the housing of the photoconductive cell becomes too dusty or dirty to pass adequate light or the light source fails.

A light source is directed at an angle against the xerographic plate so that a portion of the light is deflected off the surface of the plate into the photoconductive cell, the surface of the plate having a smooth reflective surface.

is desirable to detect the sheet 3,360,652 Patented Dec. 26, 1967 When a sheet of paper adheres to the surface of the drum, the light is deflected from the irregular surface of the paper at diiferent angles than from the drum, thus, light reaching the photoconductive cell is increased.

The intensity of the light reaching the photoconductive cell varies partially because the paper is usually of lighter color than the xerographic plate and thus reflects a larger amount of light, but also because of the diffusive nature of the surface of the paper as opposed to the reflective surface of the xerographic drum. Thus, various colored papers may be used and the photocell responds in the same manner as when white paper is used. When, for any reason, the amount of light reaching the photoconductive cell is reduced below that reflected from the surface of the xerographic drum, the photoconductive cell responds to change the characteristics of the circuit and shut down the machine. Thus, the photocell responds to three conditions or modes of operation; that is, when the machine is operating normally, the xerographic drum reflects light to the photocell and energizes a control relay which allows the machine to function in its normal manner. When a sheet of paper adheres to the xerographic drum, the photocell will detect the sheet of paper and, accordingly, change the characteristics of the electrical circuit and deenergize the control relay, shutting down the machine. The third mode of operation occurs when the photocell assembly fails to operate for any reason It is, therefore, an object of this invention to control a machine in response to increases intensity reaching a photocell;

lit is a further object of this invention to detect sheets paper which adhere to a rotating xerographic drum; It is a further object of this invention to detect misfed sheets of paper in a xerographic apparatus;

It is a further object of this invention to provide a fail safe device which detects misfed sheets of paper in a xerographic machine and also detects its own failure and shuts down the machine.

These and other objects of this invention are attained by means of a photoconductive cell responsive to increases in light reflected from misfed sheets of paper and to decerases in light resulting from failure of the photocell to receive light and the circuitry to deenergize a control relay which effects the operation of a xerographic machine.

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

and decreases in light FIG. 1 is an isometric view of a xerographic drum with a light source and a photocell mounted adjacent thereto with portions broken away to show hidden structure;

FIG. 2 is an isometric view of the light source and photocell shown in FIG. 1 with portions broken away to show internal structure;

FIG. 3 is a schematic front view of the xerographic drum and sensing box with one removed to show the internal arrangements of parts; and,

FIG. 4 is a schematic wiring diagram of a circuit for use with the photocell shown in FIG. 1.

A xerographic drum 10 is shown in FIG. 1 in the position that it would be mounted in an automatic xerographic machine. The drum is rotatably mounted on a shaft, not shown, extending through bearing 12 in the middle of the drum. The drum is held on the shaft by means of a hand nut 14, which cooperates with threads on the end of the shaft to secure the drum in place. The drum rotates in the direction shown by the arrow. Sheets of paper are fed by the conveyor 16 into surface contact with the drum cover of the sensing box and electrostatically tacked to the drum surface by transfer corotron 18. At this point xerographic powder images which have previously been developed on the drum surface are transferred to the surface of the sheet of paper by the transfer corotron 18. As the drum continues to rotate in the direction of the arrow, the sheet of paper is carried by the drum towards the conveyor belt 20. The conveyor 20 is a vacuum conveyor which will hold the paper on the surface of the belt and draws the paper away from the drum surface after the sheet has contacted the surface of the conveyor. As the leading edge of the sheet of paper rotates to a position just over the conveyor belt 20, a puffer tube 22 directs a series of sharp streams of air through nozzles 24 between the drum surface and the sheet of paper. The nozzles 24 extend across the length of the drum and effectively deflects the sheet of paper from the surface of the drum down against the surface of the belt 20. The stream of air in the putter tube 22, emerging from the nozzles 24, is produced by a small air pump, not shown, and controlled and timed by electrical circuitry, not shown, to puff at the proper time to deflect the sheet of paper onto the conveyor belt 20.

If, for any reason, the puffer mechanism fails to strip the sheet of paper from the surface of the drum, the sheet of paper would continue to rotate around the drum surface interfering with the operation of other mechanisms about the periphery of the drum. Also, with the sheet of paper on the surface of the drum, further xerographic images cannot be produced or developed; therefore, it is desirable to immediately stop the machine and manually remove the sheet of paper from the drum surface. For this purpose a photocell and lamp housing 26 is mounted adjacent to the xerographic drum immediately after the puffer tube 22 and before a precleaned corotron 28.

The photocell and lamp housing, or sensing box, 26 is seen in FIG. 2 with portions broken away to show the lamp 30 mounted in a socket 32 in one compartment of the housing 26. The lamp is angled to direct its rays through a clear or polarized glass covered opening 34 in the top of the housing against the surface of the xerographic drum 10. A second compartment 36, in the housing 26, is separated from the lamp 30 by a partition 38 and houses a photocell 40. A second opening in the top of the housing 26 immediately above the photocell 40 is covered by a clear or polarized glass 42 and permits the entrance of light rays into the housing and into the photo cell 40, as seen in FIG. 3.

The light rays from the lamp 30 are directed through the glass 34 against the surface of the drum 10, as shown by the dotted lines. Light is reflected from the surface of the drum and a portion of that light enters through the glass 42 to the photocell 40. When a sheet of paper is on the surface of the drum 10, larger amounts of light are reflected from the surface of the paper, which is rougher than the surface of the drum, through the glass 34 to the photocell 40. The changes in light intensity reaching the photocell 40 changes the characteristics of a circuit having a control relay CR-1, as shown in the schematic wiring diagram in FIG. 4. The relay CR-1 controls the operation of the automatic xerographic machine.

FIG. 4 shows a schematic wiring diagram of the circuit for the photocell 40 and the control relay CR-1. The circuit is connected to a 110-volt A.C. source. A rectifier 44 converts the current to DC. Direct current passes through a pair of resistors R1 and R2 to the photocell 40 and the control relay CR1. The photocell 40 is set for a normal mode of operation when light reflected from the surface of the xerographic drum is reflected into the photocell. A silicon controlled rectifier 46 or other threshold switching means is connected in the circuit in series with the control relay CR-l, and its gate is connected to the photocell to be controlled thereby in accordance with the current passing through the photocell.

A zener-diode 50 is connected in the circuit parallel to the control relay CR-l and the photoconductive cell 40 to maintain the voltage in the circuit at a predetermined level. Since the rectifier 44 allows current to pass on each half cycle, a condenser 52 is mounted in parallel with the control relay CR-l to provide uniform current flow through the relay on the half cycles during which no current is passing through the rectifier. A pair of silicon diodes 54 or other diodes having negative resistance conducting characteristics are connected in the triggering circuit to the gate of the silicon controlled rectifier 46 to bypass current in excess of the amount required to trigger the silicon controlled rectifier 46.

In operation, the photoconductive cell 40 is adjusted for the normal mode of operation, that is, when the normal amount of light being reflected off the surface of the xerographic drum enters the photoconductive cell the voltage and current to the gate of the silicon controlled rectifier 46 is sufficient to trigger the rectifier and permit current to pass, energizing the control relay CR-1. With the relay CR-l energized, its contacts located in the circuitry to the automatic xerographic machine are closed permitting operation of the machine. In the inoperative mode, that is, when the light bulb in the photoconductive cell fails or the glass on the photoconductive cell becomes covered with dirt suflicient to shut out light from the photoconductive cell, the resistance in the photoconductive cell 40 increases to a point wherein the voltage drop across the photoconductive cell 40 is so great that the voltage at point A is too low to trigger the silicon controlled rectifier 46. The silicon controlled rectifier opens cutting off current flow to the relay CR-l and the contacts of the relay CR-l open, shutting down the automatic xerographic machine. In the third mode of operation wherein a sheet of paper is between the photoconductive cell and the xerographic drum, that is, when the puffer misfires, the light on the photoconductive cell 40 increases and the resistance in the cell decreases to the point wherein the current flow through the cell 40 and point A increase rapidly. This current flow passes through the silicon diodes 54 and partially through the gate of the silicon controlled rectifier 46. The high current flow through the diodes 54 produces a drop in voltage at point B and through the control relay CR-l. In this mode of operation the current passing through the relay CR-1 is insuflicient to energize the relay even though the silicon controlled rectifier 46 is closed, and the contacts of the control relay CR1 again open in the automatic xerographic machine, shutting down the operation of the machine.

While the invention has been described with reference to the structure disclosed herein it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

What is claimed is:

1. A fail safe detecting device for use in detecting misfed sheets of paper in an automatic xerographic machine including:

a control means adapted to permit operation of the machine when energized and to shut down the machine when deenergized,

a threshold switching means connected in series with the control means to control current flow through the control means, a limited current power supply connected to said threshold switching means and said control means,

a photocell electrically coupled to said threshold switching means and said power supply to trigger said threshold switching means and thereby energize said control means at a photocell response of a predetermined value,

diode means coupled to said photocell and said power supply to short said power supply in response to a photocell response of higher than said predetermined value,

a light source positioned in the automatic xerographic machine to reflect a predetermined amount of light into the photocell under normal operative conditions and to reflect an increased amount of light into the photocell when a sheet of paper is misfed in the machine thereby producing a photocell response of higher than the above mentioned predetermined value,

the photocell having a resistance when exposed to light,

under normal operating conditions, capable of allowing suflicient current to pass to trigger the threshold switching means and thus allow passage of sufiicient current to energize the control means and, upon an increase of light above said predetermined amount, a resistance low enough to draw enough current to actuate said diode means so as to short said power supply and prevent operation of the control means and, upon a decrease of light, a resistance large enough to prevent passage of suflicient current to trigger the threshold switching means.

2. A fail safe device for use in xerographic reproducing machines to detect the presence of paper on a Xerographic drum and to detect its own failure to perform in the desired manner including:

a light source mounted to direct rays of light against the surface of a Xerographic drum,

a photocell mounted adjacent to the surface of a Xerographic drum in a position to receive a portion of the light rays reflected from the drum surface,

a control circuit including a control relay electrically coupled with the photocell,

the control relay being adapted to permit operation of 3,278,754 3,284,787 12/1966 Voigt et al 307-885 the Xerographic machine upon energization and to shut down the machine upon deenergization,

said circuit also having a threshold switching means the photocell being adapted to pass sufficient current to fire the threshold switching means and actuate the control relay when light is reflected from the light source ofl? the surface of the xerographic drum into the photocell and to limit the current necessary to fire the threshold switching means and thus deenergize the control relay when sufficient light fails to reach the photocell and to pass more than a predetermined amount of current upon the occurrence of a greater amount of light at the photocell than that reflected from the xerographic drum, and diode means coupled to said photocell actuated by said current of more than a predetermined amount to draw sufiicient current through the photocell to decrease the current flow through the control relay and thus deenergize said control relay so as to shut down the machine.

References Cited UNITED STATES PATENTS 10/1966 Wallace 250-2l9 WALTER STOLWEIN, Primary Examiner.

J. D. WALL, Assistant Examiner. 

1. A FAIL SAFE DETECTING DEVICE FOR USE IN DETECTING MISFED SHEETS OF PAPER IN AN AUTOMATIC XEROGRAPHIC MACHINE INCLUDING: A CONTROL MEANS ADAPTED TO PERMIT OPERATION OF THE MACHINE WHEN ENERGIZED AND TO SHUT DOWN THE MACHINE WHEN DEENERGIZED, A THRESHOLD SWITCHING MEANS CONNECTED IN SERIES WITH THE CONTROL MEANS TO CONTROL CURRENT FLOW THROUGH THE CONTROL MEANS, A LIMITED CURRENT POWER SUPPLY CONNECTED TO SAID THRESHOLD SWITCHING MEANS AND SAID CONTROL MEANS, A PHOTOCELL ELECTRICALLY COUPLED TO SAID THRESHOLD SWITCHING MEANS AND SAID POWER SUPPLY TO TRIGGER SAID THRESHOLD SWITCHING MEANS AND THEREBY ENERGIZE SAID CONTROL MEANS AT A PHOTOCELL RESPONSE OF A PREDETERMINED VALUE, DIODE MEANS COUPLED TO SAID PHOTOCELL AND SAID POWER SUPPLY TO SHORT SAID POWER SUPPLY IN RESPONSE TO A PHOTOCELL RESPONSE OF HIGHER THAN SAID PREDETERMINED VALUE, A LIGHT SOURCE POSITIONED IN THE AUTOMATIC XEROGRAPHIC MACHINE TO REFLECT A PREDETERMINED AMOUNT OF LIGHT INTO THE PHOTOCELL UNDER NORMAL OPERATIVE CONDITIONS AND TO REFLECT AN INCREASED AMOUNT OF LIGHT INTO THE PHOTOCELL WHEN A SHEET OF PAPER IS MISFED IN THE MACHINE THEREBY PRODUCING A PHOTOCELL RESPONSE OF HIGHER THAN THE ABOVE MENTIONED PREDETERMINED VALUE, THE PHOTOCELL HAVING A RESISTANCE WHEN EXPOSED TO LIGHT, UNDER NORMAL OPERATING CONDITIONS, CAPABLE OF ALLOWING SUFFICIENT CURRENT TO PASS TO TRIGGER THE THRESHOLD SWITCHING MEANS AND THUS ALLOW PASSAGE OF SUFFICIENT CURRENT TO ENERGIZE THE CONTROL MEANS AND, UPON AN INCREASE OF LIGHT ABOVE SAID PREDETERMINED AMOUNT, A RESISTANCE LOW ENOUGH TO DRAW ENOUGH CURRENT TO ACTUATE SAID DIODE MEANS SO AS TO SHORT SAID POWER SUPPLY AND PREVENT OPERATION OF THE CONTROL MEANS AND, UPON A DECREASE OF LIGHT, A RESISTANCE LARGE ENOUGH TO PREVENT PASSAGE OF SUFFICIENT CURRENT TO TRIGGER THE THRESHOLD SWITCHING MEANS. 